It is well known that color images can be obtained by exposing a silver halide color photographic light-sensitive material to light, followed by a reaction between an oxidized aromatic primary amine developing agent and a dye forming coupler. In such a process, the subtractive color process is usually adopted for color reproduction, and cyan, magenta and yellow color images, which are complementary to red, green, and blue colors, respectively, are formed. The reaction between a coupler and an oxidation product of a color developing agent proceeds at an active point of the coupler. A coupler having a hydrogen atom at its active point (i.e., a 4-equivalent coupler) stoichiometrically requires as an oxidizing agent 4 mols of a silver halide having a development center for forming 1 mol of a dye through the coupling reaction. On the other hand, a coupler having at its active point a group capable of being released in the form of an anion requires only 2 mols of a silver halide having a development center for forming 1 mol of a dye (i.e., a 2-equivalent coupler). Accordingly, by using a 2-equivalent coupler, the amount of silver halide to be used in a light-sensitive layer can be reduced and the layer per se can be made thinner, so that the time required for the processing of such a light-sensitive material can be shortened and color images obtained therefrom have an improved sharpness, when compared with a light-sensitive material in which a 4-equivalent coupler is used. In addition, the coupling activity of a 2-equivalent coupler with a color developing agent can be widely varied depending on the properties of the releasable group contained therein.
A 2-equivalent coupler capable of releasing a group having a development-inhibiting effect is called a development inhibitor releasing coupler (or DIR coupler). Such a coupler is capable of inhibiting development in proportion to the quantity of developed silver, and therefore is effective to improve graininess, gradation control, color reproducibility of images, etc. Couplers of this type can also be used in diffusion transfer processes, making use of their effects upon adjacent layers.
A 2-equivalent coupler can also be provided with a releasable group containing a diffusible dye portion. This type of couplers, which are referred to as diffusible dye-releasing couplers, can be utilized in a diffusion transfer process in which a dye image is formed from diffused dyes in an image receiving layer.
Certain colored 2-equivalent couplers have a masking effect for color correction of dye images and are usually called colored couplers.
As mentioned hereinbefore, 2-equivalent couplers can be imparted with various functions by appropriately selecting the releasable groups contained therein.
In recent development of silver halide photographic light-sensitive materials, especially, those for photographing, two outstanding trends are observed. One is the increase in sensitivity as typically shown by ASA 400 films, and another is the improvement in image quality to cope with the miniaturization of film sizes. In connection with the former, investigations have been made on a variety of techniques including large-sized silver halide grains, couplers with higher activities, acceleration of development, etc. However, increases in sensitivity based on large-sized silver halide grains seem to be reaching its limit, as reported by G. C. Farnell and J. B. Chanter in J. Photogr. Sci., 9, 75 (1961). Accordingly, this technique is not expected to make much progress in the future. In addition, the use of large-sized silver halide grains is accompanied by various disadvantages such as deterioration in graininess. Couplers having higher activities have also been studied extensively, but have not made such contribution to sensitivities of silver halide photographic light-sensitive materials, and are also disadvantageous with respect to graininess. With respect to acceleration of development, it has hitherto been attempted to incorporate various development accelerators, such as hydrazine compounds, into a silver halide emulsion layer or a developing solution mainly with regard to black-and-white photographic light-sensitive materials. However, such a technique is not practical as being often accompanied by disadvantages, such as increase in fog, deterioration in graininess, and the like.
Under such circumstances, couplers which imagewise release development accelerators or fogging agents have been proposed. For example, couplers releasing thiocyanic acid ions which accelerate the solution physical development are disclosed in U.S. Pat. Nos. 3,214,377, 3,253,924 and 4,032,345. Further, couplers releasing acyl hydrazines are described in U.S. Pat. No. 4,390,618 and couplers releasing hydroquinone or aminophenol developing agents are described in Japanese Patent Applicton (OPI) No. 138636/82 (the term "OPI" as used herein refers to a "published unexamined Japanese patent application").
U.S. Pat. No. 4,390,618 and U.S. patent application Ser. Nos. 532,631 (filed on Sept. 15, 1983) and 583,901 (filed on Feb. 27, 1984) disclose couplers releasing fogging agents and furnish details of the effects brought about by the imagewise release of the fogging agents on contrast or development acceleration. However, most of these couplers have low stability, and they undesirably cause deteriorations in photographic properties, such as a general increase in fog, desensitization during storage, and their effects reduce with the passage of time when they are incorporated into silver halide photographic light-sensitive materials. Moreover, these couplers have been found to deteriorate graininess as compared with photographic light-sensitive materials into which these couplers are not incorporated to increase contrast and sensitivity.
On the other hand, in order to improve graininess, attempts have been made to use a larger number of silver halide grains and to shade off the dye clouds formed by color development, as described in The Theory of the Photographic Process, 4th Ed., 1977, pp. 620-621, edited by T. H. James. However, to use a larger number of silver halide grains while retaining photographic sensitivity means an increased amount of silver must be applied, which results in deterioration of resolving power. Hence, such a technique is disadvantageous from the standpoint not only of cost, but also photographic properties.
Further, an attempt to improve graininess by diffusion of dyes according to the description in the above-cited James text has already been disclosed in British Pat. No. 2,080,640A. However, it has been proved that such couplers releasing diffusible dyes have reduced coupling activities, resulting in reduction of coloring density or sensitivity and increase of fog with the passage of time.
Furthermore, the above-described couplers are synthesized by introducing fogging agents into the releasable groups, and therefore the releasable groups or the coupler centers are limited so that sufficient coupling activities to attain the desired effects cannot be obtained.